CREATE OR REPLACE FUNCTION public.get_customer_balance(p_customer_id integer, p_effective_date timestamp without time zone)
 RETURNS numeric
 LANGUAGE plpgsql
AS $function$
       --#OK, WE NEED TO CALCULATE THE CURRENT BALANCE GIVEN A CUSTOMER_ID AND A DATE
       --#THAT WE WANT THE BALANCE TO BE EFFECTIVE FOR. THE BALANCE IS:
       --#   1) RENTAL FEES FOR ALL PREVIOUS RENTALS
       --#   2) ONE DOLLAR FOR EVERY DAY THE PREVIOUS RENTALS ARE OVERDUE
       --#   3) IF A FILM IS MORE THAN RENTAL_DURATION * 2 OVERDUE, CHARGE THE REPLACEMENT_COST
       --#   4) SUBTRACT ALL PAYMENTS MADE BEFORE THE DATE SPECIFIED
DECLARE
    v_rentfees DECIMAL(5,2); --#FEES PAID TO RENT THE VIDEOS INITIALLY
    v_overfees INTEGER;      --#LATE FEES FOR PRIOR RENTALS
    v_payments DECIMAL(5,2); --#SUM OF PAYMENTS MADE PREVIOUSLY
BEGIN
    SELECT COALESCE(SUM(film.rental_rate),0) INTO v_rentfees
    FROM film, inventory, rental
    WHERE film.film_id = inventory.film_id
      AND inventory.inventory_id = rental.inventory_id
      AND rental.rental_date <= p_effective_date
      AND rental.customer_id = p_customer_id;

    SELECT COALESCE(SUM(IF((rental.return_date - rental.rental_date) > (film.rental_duration * '1 day'::interval),
        ((rental.return_date - rental.rental_date) - (film.rental_duration * '1 day'::interval)),0)),0) INTO v_overfees
    FROM rental, inventory, film
    WHERE film.film_id = inventory.film_id
      AND inventory.inventory_id = rental.inventory_id
      AND rental.rental_date <= p_effective_date
      AND rental.customer_id = p_customer_id;

    SELECT COALESCE(SUM(payment.amount),0) INTO v_payments
    FROM payment
    WHERE payment.payment_date <= p_effective_date
    AND payment.customer_id = p_customer_id;

    RETURN v_rentfees + v_overfees - v_payments;
END
$function$